This invention relates to filter cartridges and a method for their production.
Such a known filter cartridge for the filtering of gases, in particular for the filtering of air is described in the German Offenlegungsschrift 19 30 715 a counterpart to U.S. Pat. No. 3,531,920. The filter cartridge comprises a filter medium which is hardenable. Furthermore, the filter medium is folded in a zig-zag fashion and is fitted in a rectangular frame. During the filtration process the medium to be filtered passes through the fold wall from one edge area of the zig-zag folding in the direction of the edge area of the folding on the opposite side. Offsets extend out on both sides of the plane of the fold walls of the zig-zag folding. The offsets in the fold walls are arranged in such a way that an offset in the form of a raised portion on one fold wall is in contact with an offset in the form of a raised portion on the adjacent fold wall. Due to the zig-zag folding the offsets of the fold walls are in contact with each other and are supported in the region of the contact area.
A drawback of this arrangement is that the filter medium comprises plastic resin or some other hardenable material. This leads to the filter characteristics of the filter medium being restricted due to the increased pressure difference which occurs due to the insertion or layering of the filter medium with the self-hardening material. The binder, in particular in the form of plastic resin, which is introduced into the filter medium so as to increase the stability thereof leads to a decrease in the permeability of the filter. Greater forces, an increased noise-level and a shorter operational life-span of the filter are the consequence. Furthermore the production of the filter medium is complicated as the binder or the plastic resin has to be introduced in addition.
A further filter medium comprising plastic fibres is described in the German Offenlegungsschrift 28 13 356 which has a high permeability but a low form stability. A drawback of this filter medium is that it cannot retain its formed shape due to the wad-like consistency and the minimal lateral and longitudinal rigidity thereof.